This invention concerns a rpm measuring apparatus having a magnetic circuit. The invention involves, more particularly, a rpm measuring apparatus having a magnetic circuit composed of a toothed disc driven by a drive shaft, a yoke carrying an induction coil, at least one portion of the yoke opposite the toothed disc being toothed in the same manner as the toothed disc.
It is known to detect the rpm of a rotating member by using a magnetic circuit of the above-described kind where the rpm-dependent magnetic flux change of the toothed wheel moving past the toothed portion of the yoke produces voltage changes having an rpm-dependent repetition rate or frequency which are used as the output signal for an electronic regulating device. This known apparatus also serves especially for the rpm-dependent regulation of injection pumps, especially fuel injection pumps. In this instance, the toothed wheel is located on the drive shaft of the injection pump and is rigidly connected therewith. The arrangement undesirably has the disadvantage that all rotational oscillations (vibrations) of the drive shaft contribute to the sensed rpm measurement as determined by the measuring apparatus and, therefore, undesirably influence the regulation. This is a special disadvantage at low rpm, for example, at idle speed, because the result is that large changes of the injection quantity corresponding to the rotational oscillations take place, and oscillations of the mechanism for changing the injection quantity of an injection pump may take place; for example, oscillations of the regulating rod of a series-type injection pump may take place.